DESCRIPTION This collaborative study will be performed in Hungary as an extension of NIH grant #RO1 DK-37021, to define the anatomical relationships between cocaine and amphetamine-regulated transcript (CART) and hypophysiotropic TRH neurons in the hypothalamic paraventricular nucleus. It is proposed that in conjunction with the parent grant, these studies will elucidate the role of CART in the regulation of hypophysiotropic TRH and determine how CART is integrated into the central control system as a mediator for the action of leptin on the hypothalamic-pituitary-thyroid axis. The origin of CART-synthesizing neurons that project to TRH neurons in the paraventricular nucleus will be identified by a two-step procedure. First, regions of the brain that contain CART-synthesizing neurons and project to the paraventricular nucleus will be identified by a double-labeling immunofluorescent technique following the stereotaxic injection of the retrogradely transported marker substance, cholera toxin subunit B (CTB), into subdivisions of the paraventricular nucleus. Second, by confocal microscopy, it will be determined whether CART-producing neurons from each of the regions where CART-IR neurons accumulate CTB, project specifically to TRH neurons in the paraventricular nucleus. This will be accomplished by a triple-labeling fluorescent technique in which the anterogradely transported marker substance, PHAL and CART will be identified by immunofluorescence in axon terminals contacting proTRH mRNA-containing neurons in the paraventricular nucleus, and proTRH mRNA will be identified by fluorescent, non-isotopic in situ hybridization histochemistry. Since CART is contained in the majority of TRH-producing neurons in the paraventricular nucleus, a double-labeling immunofluorescent technique will also be used to explore the possibility that the CART innervation to TRH neurons in the paraventricular nucleus may arise from an ultrashort feedback loop from hypophysiotropic neurons, itself. The subcellular organization of CART and TRH in these neurons will be examined by ultrastructural immunocytochemistry to determine whether both substances are packaged in the same vesicles, and whether their packaging is similarly affected by fasting. Finally, the effect of fasting and leptin administration to fasting animals on CART gene expression in hypophysitoropic TRH neurons and in CART-producing neurons that project to TRH-containing neurons in the paraventricular nucleus, will be studied using combined isotopic and non-isotopic techniques of in situ hybridization histochemistry and computerized image analysis.